1. Field of the Invention
The present invention relates to a development apparatus in which a magnet roller is arranged on the inner side of a developer bearing member and an image forming apparatus using the development apparatus.
2. Description of the Related Art
In development apparatuses of recent years, a two-component development system using a developer acquired by mixing non-magnetic toner and magnetic carrier is widely used. The two-component development system has superior stability of the charging amount of toner and thus is appropriately used by high-speed image forming apparatuses and color image forming apparatuses.
In an image forming apparatus using a two-component developer, a columnar magnet roller in which a predetermined magnetic force pattern is magnetized is arranged to be fixed inside a rotating cylindrical sleeve, and developer is attached to the surface of the sleeve according to the magnetic force pattern and is conveyed. The conveyed developer is made to be a thin layer by a regulation blade located adjacent to a developing sleeve and then, is developed in an opposing portion of a photosensitive drum. Thereafter, the developer remaining on the developing sleeve after the developing process is separated from the developing sleeve and is collected to the inside of a developing container 2.
As the magnet roller, for example, there is a magnet roller that is configured by five poles including a draw-up pole, a conveyance pole, a development pole, a collection pole, and a peel-off pole. In the case of a magnet roller configured by an odd number of poles, the draw-up pole and the peel-off pole have the same polarity, and a repulsive magnetic field is formed between both the poles. However, there are cases where a low magnetic field area attracting the developer is generated between both the poles. As such a low magnetic field area is generated, the collection rate of the developer remaining on the developing sleeve after the developing process decreases, and a history according to the developer, which has not been collected, is left after one circulation of the sleeve, whereby there is a concern that an image defection may occur.
In order to prevent this, it is necessary to control the low magnetic field area to have 10 mT or less on the surface of the sleeve, and it is necessary to form this area having 10 mT or less to be flat. In this way, the collection rate of developer is improved.
In Japanese Patent Laid-Open No. 2002-50515, the arrangement of a magnet piece that has magnetic poles of the same polarity, which have a bar shape and a cross-section having an approximate linear shape, on the side face between poles, which have the same polarity, of a magnet roller 4 (in an area in which a low magnetic field area is generated) and has an inner peripheral face having polarity opposite to that of the side face has been disclosed.
However, in the method disclosed in Japanese Patent Laid-Open No. 2002-50515, when the magnet piece is arranged for controlling the low magnetic field area, the magnetic characteristics of both end portions of the magnet roller in the longitudinal direction tend to be elevated. Thus, in both end portions of the low magnetic field area, the developer is not collected but wraps around to the outer side, whereby a stay of the developer occurs. Thus, on the surface of the developing sleeve at a position corresponding to an end face of the magnet roller in the longitudinal direction, locally, more developer than that present at a position corresponding to the circumferential face of the magnet roller is carried.
The developer staying in this way may leak to the outside of the developing container, and there is concern that the inside of the main body of the image forming apparatus is contaminated. Thus, as a measure for the leakage of the developer from the end portion of the developing sleeve, a configuration has been employed in which a magnetic plate or a magnet seal is arranged at the end portion of the developing sleeve, and a magnetic brush is brought into contact with the sleeve so as to perform magnetic sealing (see Japanese Patent Laid-Open No. 2-262171).
However, in Japanese Patent Laid-Open No. 2-262171, since the magnetic seal is necessary, the cost increases as that much. Thus, a configuration may be considered in which, by improving the magnetic characteristics of the end portion of the developing sleeve, the leakage of the developer is suppressed without using the magnetic seal.
However, in a case where the configuration not requiring the magnetic seal is employed, while the leakage of developer due to the stay of the developer in the end portion of the developing sleeve can be suppressed, there is concern about toner scattering in which toner sprouts from a gap between the developing sleeve and the developing container in the area of the end portion of the developing sleeve. The reason for this is that there is an area in which developer is scarcely present in the low magnetic field area over the entire rectangular area of the developing sleeve. In addition, since a magnetic seal is not arranged in the end portion, in the end portion of the developing sleeve, a gap, from which scattering toner leaks, is generated between the developing container and the developing sleeve.
As a measure for this, in a case where a clearance between the developing container and the developing sleeve is extremely decreased so as to fill up the gap, there is a possibility that the developing container and the developing sleeve are brought into contact with each other. In addition, as a result of the contact, there is concern that deformation of the developing sleeve according to the contact, aggregation of toner due to frictional heat generated at the time of the contact, or the like may occur.
In addition, in a conventional development apparatus, there are problems as below.
The configuration of a conventional development apparatus using two-component developer will now be described. FIG. 28 is a top view of the inside of the conventional development apparatus.
As illustrated in FIG. 28, the development apparatus 900 includes: a developing chamber 940 that supplies developer to a developing sleeve 941; and a stirring chamber 950 that is arranged to be aligned in the horizontal direction with respect to the developing chamber 940. The developing chamber 940 and the stirring chamber 950 are partitioned by a partition wall 946 having both ends open. In addition, in the developing chamber 940 and the stirring chamber 950, a conveyance screw 944 and a conveyance screw 945 that are conveyance members are respectively arranged.
By employing the configuration described above, as denoted by arrows in the figure, developer is supplied to the developing sleeve 941 while the developer is circulated between the developing chamber 940 and the stirring chamber 950 by using the conveyance screws. The developer that is carried in the developing sleeve 941 and passes through a developing area of a photosensitive body is peeled off from the developing sleeve 941 and, as denoted by broken-line arrows in the figure, is collected into the developing chamber 940.
Accordingly, in the developing chamber 940, the developer is supplied to the developing sleeve 941 while being conveyed by the conveyance screw 944, and the developer peeled off from the developing sleeve 941 is collected. For this reason, the developer disposed inside the developing chamber 940 has a more number of times of being carried in the developing sleeve 941 toward the downstream side in the conveyance direction according to the conveyance screw 944. In other words, toward the downstream side of the developing chamber 940, the developer has a more number of times of the consumption of toner by passing through the developing area.
As a result, the toner density of the developer disposed inside the developing chamber 940 decreases toward the downstream side in the conveyance direction, and non-uniformity of the toner density of the developer supplied to the developing sleeve 941 occurs. Then, when an image is formed in such a state, density non-uniformity occurs in an output object.
Meanwhile, as a development apparatus, a development apparatus has been proposed which has a configuration in which a developing chamber and a stirring chamber are arranged to be aligned in the horizontal direction, a partition wall is extended up to a position located close to a developing sleeve, and two-component developer carried on the developing sleeve is sent to the stirring chamber (see U.S. Pat. No. 6,449,442).
In the case of such a structure, developer that is peeled off from the developing sleeve after passing a developing area opposing a photosensitive body that is an image bearing member is collected into the stirring chamber. For this reason, after developer having a low toner density is sufficiently stirred in the stirring chamber, the developer is conveyed to the developing chamber and is supplied to the developing sleeve. Accordingly, it is difficult for density non-uniformity to occur in an output object.
In addition, in each of both end portions of the developing sleeve in the longitudinal direction, a developer leakage prevention member such as a magnetic plate or a magnetic sheet formed by a magnetic member having a predetermined space from a part of the circumferential face of the developing sleeve may be arranged. In such a case, two-component developer is prevented from leaking from both end portions of the developing sleeve in the longitudinal direction. In addition, a technology for preventing leakage of a magnetic carrier or non-magnetic toner by forming a magnetic brush of the two-component developer in the developer leakage prevention members has been proposed (see Japanese Patent Laid-Open No. 2-262171).
However, in the case of the development apparatus using the developer leakage prevention configuration disposed on both end portions of the developing sleeve in the longitudinal direction as disclosed in Japanese Patent Laid-Open No. 2-262171, there are problems as described below.
FIG. 29 is a diagram that illustrates an example of a development apparatus using two-component developer for describing conventional problems. In the development apparatus 1, a developing sleeve 941 that is configured using a non-magnetic material is arranged. In addition, on the inside of the developing sleeve 941, a fixed magnet 942 that is a magnetic field generation member is arranged.
When the developing sleeve 941 rotates in a direction (counterclockwise direction) denoted by an arrow X at the time of performing a developing operation, two-component developer included inside a developing container 910 is carried in the developing sleeve 941 and is conveyed in the direction X.
The magnet 942 includes five peak positions of magnetic poles. In the development apparatus of the two-component developing system, a magnet having peak positions of odd numbers is frequently used. The two-component developer included inside the developing container 910 is carried by a draw-up pole S3 to the developing sleeve 941 from the developing chamber 940.
As the developing sleeve 941 is driven to rotate in the direction X, the two-component developer is carried and conveyed and is formed in a thin layer by a regulation blade 943 arranged near a cutting pole N1. Then, a developing operation is performed for an image bearing member not illustrated in the figure near a development pole S1, and, according to a repulsive magnetic field formed by a peel-off pole S2 and the draw-up pole S3, the two-component developer is peeled off from the developing sleeve 941 and is returned to the developing chamber 940.
Between the peel-off pole S2 and the draw-up pole S3, which are the same S poles, a low magnetic force zone is generated. For this reason, while the two-component developer can be peeled off from the developing sleeve 941, in both end portions of the developing sleeve 941 in the longitudinal direction, a regulation force according to the magnetic field is weak in the low magnetic force zone between the peel-off pole S2 and the draw-up pole S3. Accordingly, the two-component developer may easily leak to the outer side of the developing sleeve in the longitudinal direction.
Thus, in Japanese Patent Laid-Open No. 2-262171, a developer leakage prevention member 949 such as a magnetic plate or a magnet sheet formed by using a magnetic member is arranged so as to at least cover the repulsive magnetic field formed by the same poles. In this way, the two-component developer is prevented from leaking from both end portions of the developing sleeve 941 in the longitudinal direction.
However, in a case where the developer leakage prevention member 949 such as the magnetic plate or the magnetic sheet is arranged in a repulsive magnetic field, the cost increases by arranging the developer leakage prevention member 949. In addition, in order to improve the sealing property, a higher-priced developer leakage prevention member is required, and there is concern that the cost further increases. Particularly, in the case of a configuration in which developer is circulated between the developing chamber and the stirring chamber, the developer is configured to leak from the end portion more easily on the downstream side in the developer conveyance direction than on the upstream side according to the relation of developer pressure.
This problem similarly occurs also in a development apparatus having the configuration, as illustrated in U.S. Pat. No. 6,449,442, in which the developing chamber and the stirring chamber are arranged to be aligned in the horizontal direction, the partition wall is extended up to a position located close to the developing sleeve, and two-component developer carried on the developing sleeve is sent to the stirring chamber.
In addition, in the conventional development apparatus, there are problems as below.
As a development apparatus using an electrophotographic system, there are development apparatuses each using two-component developer including non-magnetic toner particles (toner) as developer and magnetic carrier particles (carriers). In such development apparatuses, when developer leaks from an end portion of a developing sleeve, there is concern that a bearing as a bearing member of the developing sleeve is clogged, and defective rotation of the developing sleeve occurs.
Thus, it is known that a magnetic member is used so as to surround a developer bearing member in the peripheral direction except for an area corresponding to an opening portion of a developing container in an end portion of the developing sleeve in the direction of a rotation shaft of the developing sleeve as a developer seal (see Japanese Patent Laid-Open No. 2-262171).
In Japanese Patent Laid-Open No. 2-262171, as illustrated in FIG. 1, the magnetic member 21 covers an end portion of an area other than a developing area in which the developing sleeve is exposed from the development apparatus so as to face an image bearing member. In other words, the magnetic member 21 covers an end portion of an area other than a developer area including portions in which a pole N2 taking in the developer and a pole N3 peeling off the developer are present. Accordingly, the two-component developer is prevented from leaking from both end portions of the developing sleeve in the longitudinal direction.
In the magnetic member, since the pole taking in the developer and the pole N3 peeling off the developer have the same polarity, between the poles having the same polarity, a low magnetic field area is formed. Then, in an end portion of the low magnetic field area, there is a high possibility of the occurrence of developer leakage, and accordingly, it is necessary to cover all the areas other than the developing area with the magnetic member as in Japanese Patent Laid-Open No. 2-262171.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 2-262171, there is a disadvantage that, when the developer after the developing process is put into the inside of the developing container from a developer taking portion, the developer seal disposed in the end portion of the developer bearing member hinders the flow of the developer in the developer taking portion.
The hindrance of the flow of the developer causes the developer to stay in the end portion of the developer taking portion. Then, there is concern that toner included in the developer disposed in the stay portion is separated, and toner scattering occurs inside an image formation apparatus. In addition, there is concern that the developer overflown from the stay portion falls into the inside of the image forming apparatus. Furthermore, the scattering toner or the fallen developer, finally, may have a bad influence such as contamination on an output image.
In addition, in the conventional development apparatus, there are also problems as below.
In image forming apparatuses of recent years, a two-component developing system using two-component developer acquired by mixing non-magnetic toner and magnetic carriers together is widely used. In the image forming apparatuses using the two-component developer, a columnar magnet roller in which a predetermined magnetic force pattern is magnetized is arranged to be fixed inside a rotating cylindrical sleeve. Then, developer is attached to the surface of the developing sleeve according to the magnetic force pattern of the magnet roller and is conveyed.
As the magnet roller, for example, there is a magnet roller that is configured by five poles including a draw-up pole, a conveyance pole, a development pole, a collection pole, and a peel-off pole. In the case of a magnet roller configured by an odd number of poles, the draw-up pole and the peel-off pole have the same polarity, and, by forming a low magnetic field area between both the poles, the developer after the developing process is peeled off from the developing sleeve.
However, generally, the magnetic characteristics of both end portions of the magnet roller in the longitudinal direction tend to be elevated. Thus, there are cases where, in both end portions of the sleeve, the developer is not collected but wraps around to the outer side, and a stay of the developer occurs. In other words, on the surface of the sleeve at a position corresponding to an end face of the magnet roller in the longitudinal direction, locally, more developer than that present at a position corresponding to the circumferential face of the magnet roller is carried.
As a result, in the portion in which more developer is carried, there is concern that the applied pressure of the regulation blade increases, and deterioration of the developer due to sliding friction progresses, or linear development non-uniformity on the image face is generated. In addition, there is concern that the staying developer may leak to the outside of the developing container.
Thus, in order to reduce the elevation of the magnetic force on the end face of the magnet roller in the longitudinal direction, chamfering is performed for the end portion of the columnar magnet roller (see Japanese Patent Laid-Open No. 10-091002). In such a configuration, the diameter of the end portion of the magnet roller is gradually decreased toward the outer side in the longitudinal direction, and a distance between the surface of the magnet roller and the surface of the sleeve is gradually increased. In this way, the elevation of the magnetic force at the corner of the end portion of the magnet roller is offset.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 10-091002, while there is an advantage of not requiring a magnetic seal, there is concern that the developer leaks due to vibrations at the time of shipping.